<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <div class="box">盒子1</div>
  <div class="box">盒子2</div>
  <div id="nav">
    <ul>
      <li>首页</li>
      <li>产品</li>
    </ul>
  </div>
  <script>
    // 1. getElementsByClassName 根据类名获得某些元素集合
    var boxs = document.getElementsByClassName('box');
    console.log(boxs);
    // 2. querySelector 返回指定选择器的第一个元素对象,切记，里面的选择器需要加符号 .box(class)  #nav(id)
    var firstBox = document.querySelector('.box');
    console.log(firstBox);
    var nav = document.querySelector('#nav');
    console.log(nav);
    var li = document.querySelector('li');
    console.log(li);

    // 3.  返回指定选择器的所有元素对象
    var allBox = document.querySelectorAll('.box');
    console.log(allBox);
    var lis = document.querySelectorAll('li');
    console.log(lis);
  </script>
</body>

</html>